Perpendicularly magnetized MTJs (p-MTJs) are a major emerging technology for use as embedded magnetic random access memory (MRAM) applications, and standalone MRAM applications. P-MTJ MRAM technology using spin-torque (STT-MRAM) for writing of memory bits was described by C. Slonczewski in “Current driven excitation of magnetic multilayers”, J. Magn. Magn. Mater. V 159, L1-L7 (1996), and is highly competitive with existing semiconductor memory technologies such as SRAM, DRAM, and flash.
Reducing the critical switching current density for p-MTJs is a key challenge for integrating MRAM and STT-MRAM into existing complementary metal oxide semiconductor (CMOS) technologies. As the write current is reduced, smaller transistors may be used for each bit cell thereby potentially enabling higher density memory arrays and lower production cost. One of the strategies explored in the past for minimizing the critical current (ic) for switching the free layer in a p-MTJ is a dual spin filter structure also referred to as a DMTJ. A typical DMTJ has a PL1/TB1/FL/TB2/PL2 configuration wherein PL1 and PL2 are first and second pinned layers, that adjoin first and second tunnel barrier layers TB1 and TB2, respectively, and create a spin torque effect on the free layer (FL) when a current is passed through the DMTJ in a perpendicular to plane direction. Preferably, each of PL1, PL2, and the FL has a magnetization aligned in a perpendicular to plane (vertical) direction. When PL1 and PL2 are initialized anti-parallel to each other, there is potentially a two-fold increase in the spin torque on the FL compared with a MTJ having a single spin polarizer in a PL/TB/FL configuration, for example. As a result, there is improved spin torque transfer efficiency and a reduction in (ic).
In the aforementioned DMTJ, the PL1/TB1/FL stack may be considered as a first p-MTJ substructure while the FL/TB2/PL2 stack may be considered as a second p-MTJ substructure. In each p-MTJ substructure, the FL is free to rotate to a direction that is parallel (P state) or antiparallel (AP state) with respect to PL1 and PL2. It is important for the net magnetoresistive ratio (DRR) to be a large value, preferably higher than 1, as DRR is directly related to the read margin.
The magnetic performance for a DMTJ with two p-MTJ substructures is related not only to DRR and ic, but also to the difference (RA2−RA1). In the prior art, one or two of these parameters are addressed with a new design, but there is a need to optimize all three simultaneously, and to reduce ic lower than that achieved with a single p-MTJ cell. Therefore, an improved DMTJ structure is needed where ic is minimized without a substantial sacrifice in DRR, and without increasing RA to an unacceptably high level that would lead to a decreased lifetime for one or both tunnel barrier layers.